In some wireless communication systems, a user equipment (UE) wirelessly communicates with a base station to send data to the base station and/or receive data from the base station. A wireless communication from a UE to a base station is referred to as an uplink communication, and a wireless communication from a base station to a UE is referred to as a downlink communication.
Resources are required to perform uplink and downlink communications. For example, a UE may wirelessly transmit data to a base station in an uplink transmission at a particular frequency and/or during a particular slot in time. The frequency and time slot used are examples of resources.
In some wireless communication systems, if a UE is to transmit data to a base station, the UE requests uplink resources from the base station. The base station grants the uplink resources, and then the UE sends the uplink transmission using the granted uplink resources. An example of uplink resources that may be granted by the base station is a set of time/frequency locations in an uplink orthogonal frequency-division multiple access (OFDMA) frame.
The base station is aware of the identity of the UE sending the uplink transmission using the granted uplink resources, because the base station specifically granted those uplink resources to that UE. However, there may be schemes in which the base station does not know which UE, if any, is going to send an uplink transmission using certain uplink resources. An example is a grant-free uplink transmission scheme in which UEs may send uplink transmissions using certain uplink resources shared by the UEs, without specifically requesting use of the resources and without specifically being granted the resources by the base station. The base station will therefore not know which UE, if any, is going to send a grant-free uplink transmission using the resources. Also, assuming there is no coordination amongst the UEs, then two or more UEs may each send a respective grant-free uplink transmission using the same resources, causing a collision.
A Grant-Free (GF) transmission refers to a transmission in a communication system using communication resources for which an explicit grant of access to use those resources is not required. GF transmission is a promising technology that may enable services with very tight latency, such as ultra-Reliable Low Latency Communications (uRLLC), and may provide reduced scheduling overhead and energy savings for applications such as massive Machine-Type Communication (mMTC).
One important feature of GF transmission is that GF communication resources that are used for GF transmission are not scheduled by a communication network. UEs can transmit over GF communication resources without any grant from the network.